1. Field of the Invention
The present invention relates to a multistage turbine with a single blade row and a gas turbine, and more particularly to a high-efficiency multistage turbine with a single blade row for desirably preventing leakage of a working fluid from the tip clearance to reduce loss of a heat drop of the working fluid, and a gas turbine using the multistage turbine with a single blade row.
2. Description of the Related Art
The driving mechanism of fans in a general turbine engine is a low pressure turbine which is connected to the rotation axis of a fan. However, as another fan driving method, there is a tip turbine system in which a small turbine rotor blade (referred to as “tip turbine” hereafter) is embedded in a leading end of a fan. In this tip turbine system, a tip turbine is embedded in a circumferential end portion of a fan, thus there are advantages that a rotation axis for connecting a low pressure turbine to the fan is not required, the shaft length of the engine is reduced by the low pressure turbine moving to the circumferential end portion of the fan, and the structure of the engine is significantly simplified.
On the other hand, the hub diameter of the tip turbine inevitably tends to become larger than the diameter of the fan, and the flow passage area of the tip turbine also tends to become large. Therefore, when flow rate of the working fluid for driving the tip turbine is relatively small in proportion as the flow passage area, the height of a tip turbine rotor blade needs to be reduced. However, a radical reduction of the height of the rotor blade causes degradation of performance of the blade, thus conformity of the amount of the working fluid flowing inside the turbine with the flow passage area of the turbine cannot be achieved, a partial turbine or the like in which a tip turbine is disposed in a part of the circumference of fan is used.
Further, circumferential speed of the tip turbine significantly depends on circumferential speed of the fan. Especially when the tip turbine comprises only a single rotor blade row, desired circumferential speed cannot be obtained, and a heat drop required for the tip turbine to drive the fan cannot be absorbed. For this reason, there is known a multistage turbine with plurality of blade rows in which the tip turbine comprises a plurality of rotor blade rows (see, for example, Japanese Patent Application Laid-Open No. H6-272619). Alternatively, when the pressure of the gas for driving the tip turbine rises, the partial tip turbine with multistage rotor blade rows is used.
Incidentally, there is known a multistage turbine with a single blade row, in which the rotor blade row of the tip turbine, without being changed, transmits a working fluid repeatedly, and thereby subjecting the working fluid to adiabatic expansion throughout the multiple stages to absorb a desired heat drop (see, for example, Japanese Patent Application Laid-Open No. 2003-293702). This multistage turbine with a single blade row comprises a mechanism for transmitting the working fluid through the single blade row a number of times, and brings about an operational effect of a multistage turbine, although it has only a single blade row. Therefore, the structure thereof is simpler than when a tip turbine is made to have a plurality of blade rows and multiple stapes, and also its weight can be reduced.
In the above multistage turbine with a single blade row the working fluid is subjected to adiabatic expansion each time when it passes through a row of rotor blades, and its pressure is reduced while applying work to the rotor blade row. For this reason, working fluids of different pressures at both sides of a rotor blade may sometimes flow parallel to each other.
However, when a working fluid of high pressure and a working fluid of low pressure flow parallel to each other, a large quantity of the high-pressure working fluid flows into the low-pressure working fluid via a space between rotor blades (referred to as “tip clearance” hereinafter), whereby a heat drop, which is obtained when a working fluid is subjected to adiabatic expansion when it passes through a rotor blade row, cannot be recovered sufficiently, causing a problem that performance of the tip turbine and therefore performance of the entire blades are degrade.
On the other hand, leakage of the working fluid through the tip clearance largely depends on the conditions of the pressure at downstream of the rotor blade and on the pressure gradient between the rotor blades. Particularly, a rotor blade, which is positioned in a section which starts receiving the high-pressure working fluid from the low-pressure working fluid as this blade rotates, receives high pressure from the suction side of the blade gradually, thus the pressure at the suction side of the blade becomes higher than that at the pressure side of the blade. At this moment, when the downstream of the suction side of the blade is exposed to the working fluid, the fluid on the suction side of the blade tries to flow toward the pressure side of the blade where the pressure is low, and then flow converges into the tip clearance. As a result, a large quantity of the high-pressure working fluid starts to flow toward the pressure side of the blade from the suction side of the blade via the tip clearance, whereby the performance of the blade is degraded significantly. Also, regarding the rotor blade, which is positioned in a section which starts receiving the low-pressure working fluid from the high-pressure working fluid as this blade rotates, a large quantity of the high-pressure working fluid starts to flow toward the suction side of the blade from the pressure side of the blade via the tip clearance for the same reason, whereby the performance of the blade is degraded significantly.
Incidentally, although the above problems can reduce the effect on the rotor blades by increasing the number of the rotor blades, they cause reduction of the chord length, but a radical reduction of the chord length inhibits the aerodynamic function of the turbine.